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Zeb,
You will not get agreement on the answer to your question.
Some use ER70 others use ER80. On the frame itself I doubt
you'll be able to do post weld heat treating.
I use ER80 and don't believe in the practice of torch annealing
the welded joints. I do use lowest heat needed an don't "hurry"
the weld along.
Opinions vary....do lots of reading and make your choice.
Will also mention that the Lincoln motorsports welding program
falls in the ER80 camp.
Dave P.

ER80S-D2 is a higher strength steel filler that contains 1/2% of Molybdenum and more closely matches the strength of the 4130 than the ER70. You can stress relieve the weld joint if you wish but even some of the top drag chassis builders don't do it. Heat treating is only somewhat useful and relies on how much base metal is diluted with the filler. Normally, heat treating is done with 4130 filler and on thicker weldments. Keep your arc lengths as close to the joint as you can and do not overheat by using too much amperage but make sure the toes of the weld are tied in properly. With a mini-sprint chassis and the amount of triangulation that is used in the tube layout, you should be fine.

The important point here is that on a race vehicle, the design is light weight, close to the limit of strength / failure. Some, but not a lot of "extra" safety margin.

Sooooo, what that means is, you race, you inspect, you race, you inspect, repeat until the frame is wore out...

Where folks get into trouble is they under design, never inspect, or forget that life cycle deal.

If you want to be competative, weight is an issue, having a vehicle break up under you or worse yet, not protect you when you really need it like when you tangle with a wall or 3 or 4 other vehicles.

Just kiddding about the coat hangers by the way.

Coat hangers??? where the heck are you finding metal ones now-days??! We always called them RG-Black. However if given the choice between a coat hanger from 1940, and filler metal from china.......I would trust the coat hanger to at least be consistent!